Adaptive Cruise Control Design draft1p3

1 1 5th Meeting of the Software System Safety Working Group April 12th-14th 2005 @ Anaheim, California USA Adaptive Cruise Control System Overview 1 Introduction Adaptive Cruise Control (ACC) is an automotive feature that allows a vehicle's Cruise Control system to adapt the vehicle's speed to the traffic environment. A radar system attached to the front of the vehicle is used to detect whether slower moving vehicles are in the ACC vehicle's path. If a slower moving vehicle is detected, the ACC system will slow the vehicle down and Control the clearance, or time gap, between the ACC vehicle and the forward vehicle.

2 If the system detects that the forward vehicle is no longer in the ACC vehicle's path, the ACC system will accelerate the vehicle back to its set Cruise Control speed. This operation allows the ACC vehicle to autonomously slow down and speed up with traffic without intervention from the driver. The method by which the ACC vehicle's speed is controlled is via engine throttle Control and limited brake operation. 2 Definitions and Physical Overview ACC VehicleTarget VehicleForward Vehicleclearance(time gap = clearance / vehicle speed) Figure 1 ACC Vehicle Relationships 2 Definitions Adaptive Cruise Control (ACC) An enhancement to a conventional Cruise Control system which allows the ACC vehicle to follow a forward vehicle at an appropriate distance.

3 ACC vehicle the subject vehicle equipped with the ACC system. active brake Control a function which causes application of the brakes without driver application of the brake pedal. clearance distance from the forward vehicle's trailing surface to the ACC vehicle's leading surface. forward vehicle any one of the vehicles in front of and moving in the same direction and traveling on the same roadway as the ACC vehicle. set speed the desired Cruise Control travel speed set by the driver and is the maximum desired speed of the vehicle while under ACC Control .

4 System states ACC off state direct access to the 'ACC active' state is disabled. ACC standby state system is ready for activation by the driver. ACC active state the ACC system is in active Control of the vehicle's speed. ACC speed Control state a substate of 'ACC active' state in which no forward vehicles are present such that the ACC system is controlling vehicle speed to the 'set speed' as is typical with conventional Cruise Control systems. ACC time gap Control state a substate of 'ACC active' state in which time gap, or headway, between the ACC vehicle and the target vehicle is being controlled.

5 ACC offACC standbyACC speedcontrolACC time gapcontrolACCactiveACC onACC offactivatede-activateACC off Figure 2 ACC States and Transitions 3 target vehicle one of the forward vehicles in the path of the ACC vehicle that is closest to the ACC vehicle. time gap the time interval between the ACC vehicle and the target vehicle. The 'time gap' is related to the 'clearance' and vehicle speed by: time gap = clearance / ACC vehicle speed Physical Layout As shown in Figure 3, the ACC system consists of a series of interconnecting components and systems.

6 The method of communication between the different modules is via a serial communication network known as the Controller Area Network (CAN). ACC Module The primary function of the ACC module is to process the radar information and determine if a forward vehicle is present. When the ACC system is in 'time gap Control ', it sends information to the Engine Control and Brake Control modules to Control the clearance between the ACC Vehicle and the Target Vehicle. Engine Control Module The primary function of the Engine Control Module is to receive information from the ACC module and Instrument Cluster and Control the vehicle's speed based on this information.

7 The Engine Control Module controls vehicle speed by controlling the engine's throttle . Brake Control Module The primary function of the Brake Control Module is to determine vehicle speed via each wheel and to decelerate the vehicle by applying the brakes when requested by the ACC Module. The braking system is hydraulic with electronic enhancement, such as an ABS brake system, and is not full authority brake by wire. Instrument Cluster The primary function of the Instrument Cluster is to process the Cruise Switches and send their information to the ACC and Engine Control Modules.

8 The Instrument Cluster also displays text messages and telltales for the driver so that the driver has information regarding the state of the ACC system. CAN The Controller Area Network (CAN) is an automotive standard network that utilizes a 2 wire bus to transmit and receive data. Each node on the network has the capability to transmit 0 to 8 bytes of data in a message frame. A message frame consists of a message header, followed by 0 to 8 data bytes, and then a checksum. The message header is a unique identifier that determines the message priority.

9 Any node on the network can transmit data if the bus is free. If multiple nodes attempt to transmit at the same time, an arbitration scheme is used to determine which node will Control the bus. The message with the highest priority, as defined in its header, will win the arbitration and its message will be transmitted. The losing message will retry to send its message as soon as it detects a bus free state. Cruise Switches The Cruise Switches are mounted on the steering wheel and have several buttons which allow the driver to command operation of the ACC system.

10 The switches include: 'On': place system in the 'ACC standby' state 'Off'': cancel ACC operation and place system in the 'ACC off' state 4 'Set +': activate ACC and establish set speed or accelerate 'Coast': decelerate 'Resume': resume to set speed 'Time Gap +': increase gap 'Time gap ': decrease gap InstrumentClusterCruise SwitchesEngineControlModuleACC Modulewith RADARB rakeControlModuleBrakeSwitchesBS1BS2 ControllerAreaNetwork(CAN)Brake LightsBrake LightsText DisplayWheel SpeedWheel SpeedWheel SpeedWheel SpeedHydraulic Brake LineHydraulic Brake LineHydraulic Brake LineHydraulic Brake Line Figure 3 Physical Layout 5 Brake Switches There are two brake switches, Brake Switch 1 (BS1) and Brake Switch 2 (BS2).